At – This preposition of time is used to discuss
clock times,
holidays and festivals, and other very specific time frames including
exceptions, such as “at night.”
On – This preposition of time is used to discuss certain days of the week or portions of days of the week, specific dates, and special days such as “on New Year’s Day.”
In – This preposition of time is used to discuss months, seasons, years, centuries, general times of day, and longer periods of time such as “in the past.”